The use of mobile radio communications continues to grow rapidly and new data services are becoming increasingly popular. Many of these new services put greater requirements on radio quality than traditional speech services at the same time as speech traffic volumes continue to rise. Given the limited radio spectrum typically available to network operators, techniques that can increase the spectral efficiency of cellular networks are needed. Moreover, to achieve optimal performance, systems must be able to manage a range of services and terminals with varying radio requirements and interference suppression capabilities in a highly efficient manner.
An exemplary cellular network technology is GSM whose traffic loads are currently more intense than in any other type of system. With the non-uniformity of many commercial networks, extreme traffic peaks can occur in certain areas at busy hour and cells requiring close to 100% hardware load, with as many installed transceivers as there are available frequencies, already exist. Moreover, adaptive antenna products that can enable an additional doubling of spectral efficiency will soon be available [1]. Hence, the means for handling extreme loads, including the reuse of radio resources within cells, may not just be an issue for future networks using GSM or other technologies; solutions could be needed for hot spot areas in existing networks relatively soon.
Very high spectral efficiency and the means for handling extreme loads can be provided by the Channel Allocation Tiering (CHAT) technique [2]. CHAT decouples the hardware resources of a cell from the radio resources and it is possible to install a larger number of hardware resource units, e.g. transceivers, than available radio resources, e.g. frequencies, when necessary. Intra-cell co-channel interference will typically be present at peak loads in a CHAT configuration, but CHAT provides several mechanisms to manage this. Adaptive narrow-beam antennas, intra-cell interference diversity through radio resource hopping, and advanced interference suppression receivers are examples of mechanisms that may be employed.
CHAT uses fractional reuse of radio resources, that is, reuse less than one. This means that different terminals in the same cell or cell sector can use the same radio resource at the same time.
CHAT is also disclosed in [3] and may be described in the following simplified and incomplete, but illustrative manner with reference to FIG. 1 which illustrates a number of hardware resource units 1a-h, 2a-h, for example transceivers, a number of radio resources 3a-h, for example frequencies, and a cell 4 in which the resources 1-3 are available.
According to the CHAT technique the available hardware resource units are split into a number of logical groups, channel tiers 5 and 6, each of a maximum size h, where h is an integer. This maximum size h equals the number of radio resources allocated to each cell. The cell can be regarded as being comprised of two logical cells 7, 8 each corresponding to a respective tier. The fractional reuse in CHAT is achieved by sharing the radio resources among different channels in different tiers. In the example shown the available hardware resource units are split into two tiers. The two tiers utilize the same radio resources 3a-h. For example, two channels, one (1a) from tier 5 and one (2a) from tier 6, share the same radio resource 3a at this moment in time. In general, all tiers partially or fully share the same radio resources. However, sharing is very flexible. A channel may share its radio resources with other channels in different tiers, or it may use its allocated radio resources by itself without sharing. When two channels share the same radio resource, a collision occurs and intra-cell co-channel interference is generated. The effects of this should be minimized. In CHAT a number of techniques may be used to achieve the lowest possible collision rate and to minimize the effects of any collisions that do occur. Such techniques include different radio resource hopping sequences, e.g. frequency hopping sequences, in the two tiers, different training sequences in the two tiers together with interference suppression in the receivers, and various interference avoidance techniques such as spatial separation using adaptive antennas.
CHAT is preferably used when the number of radio resources is less than the number of hardware resource units in the cell. However, CHAT may be used in cells where the number of radio resources is more than the number of required hardware resource units.
A characteristic of the CHAT technique is that the amount the channel tiers interfere with one another is dependent on the traffic load in each tier. The allocation of traffic between the tiers will therefore affect the interference levels in the other tiers. To explain this, suppose traffic load in tier 5 is high, and low in tier 6. Since traffic load in tier 5 is high, the number of terminals in tier 5 is larger than in tier 6 where the traffic load is less. Terminals that are allocated channels in tier 6 will therefore experience more intra-cell co-channel interference (from the many terminals in tier 5) than those in tier 5 which are only affected by the few terminals in tier 6. Intra-cell co-channel interference only occurs between, not within, different tiers.
In accordance with [3] the hardware resource units in prior art CHAT systems are divided into tiers when the number of needed hardware resource units is greater than the number of radio resources in that same cell. In the following this tier configuration process is referred to as slow CHAT configuration. In accordance with [3] channel allocation to the different tiers is made with consideration to the current load in the tiers. In the following channel allocation to the different tiers is referred to as fast channel allocation. Reference [3] is silent on the means and measures to be used for the fast channel allocation.
One possible way to obtain an even spread of intra-cell interference between users in a cell or cell sector would be to divide the hardware resource units into equally sized channel tiers whenever a second (or higher) tier is introduced. However, this means that intra-cell co-channel interference will occur at far lower radio resource loads than necessary, i.e. much lower than 100%, which is clearly undesirable. Another possible way to obtain an even spread of intra-cell interference between users in a cell would be to allocate channels randomly. This, however, may have the implication that a channel in a more highly interfered channel tier is allocated to a terminal on which a service is running, that has high radio requirements in terms of the carrier-to-interference ratio (C/I), bit error rate, quality-of-service (QoS) etc., with the result that the terminal will experience excessive intra-cell co-channel interference. Another implication may be that a channel in a more highly interfered channel tier is allocated to a terminal which resides in a bad radio location, for example a terminal at the border of a cell, with the result that the terminal will experience excessive intra-cell co-channel interference. Still another implication may be that a channel in a more highly interfered channel tier is allocated to a terminal that has no or only weak interference suppression capabilities with the result that the terminal will experience excessive intra-cell co-channel interference. Intra-cell co-channel interference problems become even worse if a channel in a more highly interfered channel tier is allocated to a terminal that both has high radio requirements and a bad radio location, or is allocated to a terminal in a situation which is a combination of the listed conditions.
In [4] a radio communications system is described that utilizes multiple code sets in the downlink of a CDMA system to allow more connections in a cell. Codes are allocated to different users taking into account the interference between them.